Residual oxygen consumption

Description

Residual oxygen consumption, ROX, is respiration due to oxidative side reactions remaining after inhibition of the Electron transfer-pathway (ET-pathway) in mitochondrial preparations or cells, or in mt-preparations incubated without addition of fuel substrates (in the presence of ADP following a stimulation of the consumption of endogenous fuel substrates: State 2). Different conditions designated as ROX states (different combinations of inhibitors of CI, CII, CIII and CIV; or respiration of mt-preparations without addition of fuel substrates) may result in consistent or significantly different levels of oxygen consumption. Hence the best quantitative estimate of ROX has to be carefully evaluated. Mitochondrial respiration is frequently corrected for ROX as the baseline state. Then total ROUTINE, LEAK, OXPHOS or ET-pathway (R, L, P and E) respiration is distinguished from the corresponding ROX-corrected, mitochondrial (ET-pathway linked) fluxes: R(mt), L(mt), P(mt) and E(mt). When expressing ROX as a fraction of ET-capacity (flux control ratio), total flux, E (not corrected for ROX), should be taken as the reference. ROX may be related to, but is of course different from ROS production.
» MiPNet article

Abstract: Residual oxygen consumption (ROX) is sometimes referred to as 'non-mitochondrial respiration'. This may be correct to a large extent, but is not entirely accurate. In a preparation of purified isolated mitochondria, a small but significant ROX is observed after appropriate correction for instrumental background oxygen flux. In this case, ROX is 'mitochondrial non-ET-pathway' rather than ‘non-mitochondrial’ respiration. In permeabilized and intact cells, ROX may be higher than in isolated mitochondria, and this increased part then would be a measurement of non-mitochondrial respiration. Keilin (1926) introduced and accurately defined the term residual respiration.

Mitochondrial physiology nomenclature: Despite the fact that the previously used system of abbreviations is linked to a conceptually consistent framework of physical chemistry, generalized applicability may gain from a shift to a pragmatic emphasis of the experimental context of mitochondrial physiology. ROX correction is a baseline correction, which may be evaluated by a direct measurement within an experimental assay, or by independent test experiments for critical evaluation of the best estimate for a particular ROX correction. The ROX correction, therefore, may be emphasized by labelling the ROX-corrected values as mitochondrial (mt) fluxes: R(mt), L(mt), P(mt) and E(mt). Unfortunately, the total R, L, P and E values in this mitochondrial physiology system carry opposite meanings to the physical chemistry system of terminology. Confusion may be avoided in this context by explicit abbreviations, R(total), L(total), P(total) and E(total). Such lengthy abbreviations are not really practical.

Agreement is required for paving the way towards consistent data reporting and managing a meaningful database (but agreement is not even reached in hot debates on caloric units [J versus cal] or frozen debates on commonly used temperature scales [°C versus Faraday]. Unfortunately, the two systems of abbreviations for ROX correction are not compatible. It is of practical importance to use short and well defined symbols. As a practical guideline, therefore, the simplest solution is to define in a given context, if data (and symbols used) refer to total or ROX-corrected results, and then use the simple symbols R, L, P and E in such a defined context.

Keilin 1929: Residual respiration

'KCN, H2S and CO combine with some of the components of oxidase forming an inactive compound, with the result that cytochrome, or at least its components a’ and c’, as well as paraphenylenediamine added to the cells, are not oxidised. The respiratory process can be still carried out through the medium of some autoxidisable carriers such as haemochromogens, haematins, the component b’ of cytochrome, or some as yet unknown autoxidisable substances. This residual respiration, according to the nature of the cell, may represent a larger or smaller fraction of the total respiration of the cell.'

Experimental tests on ROX

Is the preparation fully permeabilized?

ROX may be estimated in mitochondrial preparations in SUIT protocols which start at a respiratory state without added CHO substrates and without inhibitors, such that continuation of the SUIT protocol is possible for OXPHOS analysis. Some critical questions arise for evaluation, if this respiratory 'ROX state' represents a valid condition for estimation of ROX, and for comparison with ROX measured at the end of a SUIT protocol after titration of substrates and inhibitors of key elements of the Electron transfer-pathway (e.g. Rotenone, Malonate and Antimycin A as inhibitors of CI, CII and CIII). For evaluation of such an initial estimate of ROX, the following considerations are suggested.

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